1. Field of the Invention
The present invention relates to an optical phase shifter changing the optical path length of transmitted light by a thermo-optical effect and a demodulator for an optical phase modulation signal which uses the optical phase shifter.
2. Description of the Related Art
In recent years, phase modulation methods have been put to practical use in response to demands for a large capacity and a long range of an optical transmission system. For example, in a phase modulation method such as differential phase shift keying (hereinafter, referred to as DPSK) or differential quadrature phase shift keying (hereinafter, referred to as DQPSK), when a transmitted optical signal is received, demodulation is performed by making the signal optically interfere with an optical signal before one bit and converting phase information into intensity information.
A technique associated with an optical interferometer is disclosed in JP 6-21891 A. As shown in FIG. 1 of JP 6-21891 A, an optical signal transmitted by an optical transmitter is input so as to be branched to two optical paths having different optical path lengths, the branched optical signals are recombined again, the optical signals are interfered by each other due to an effective optical path difference of the optical paths, so that the optical signal is converted into an intensity modulation signal in the optical interferometer. Then, an optical receiver converts the intensity modulation signal converted by the optical interferometer into an electric signal, an amplifier amplifies the electric signal converted by the optical receiver, and a signal processing unit extracts a data signal from the electric signal amplified by the amplifier. In order to demodulate the optical signal with high accuracy, it is necessary to accurately set a delay time granted to one of the optical signals branched in the optical interferometer. As shown in FIG. 2 of JP 6-21891 A, a phase shift unit of the optical interferometer shifts the optical path length.
As means for adjusting the optical path length, there are generally known methods of using physical optical phenomena such as an electro-optical effect, a magneto-optical effect, a photo-elastic effect, and a thermo-optical effect or methods of mechanically moving an optical element.
In the related arts, many optical interferometers are configured by a planar light-wave circuit (hereinafter, referred to as PLC). However, the PLC has the feature in which optical waveguide characteristics are highly sensitive to changes of temperature or mechanical pressure. For this reason, a problem may arise in that cost increases and the size of the PLC increases to stably maintain the optical waveguide characteristics. JP 2003-287632 A discloses a waveguide type optical module including a waveguide element such as a quartz-array waveguide element in which branched-wave wavelength characteristics change dependently on temperature and a temperature control element on which the waveguide element is placed. The temperature control element disclosed in JP 2003-287632 A is configured by a plate-like body that includes a heating member on a surface opposite to a surface on which the waveguide element is placed or in the inside thereof. By decreasing the area in which the temperature control element and a pedestal (external unit) supporting the temperature control element physically have contact with each other and interposing a heat insulator between the temperature control element and the pedestal, heat propagation from the temperature control element (plate-like body) to the pedestal is reduced, thereby ensuring thermal uniformity of a plate surface.
For example, WO 2010-109640 A1 discloses a delay interferometer that uses an optical system (hereinafter, referred to as a free space optical system) using a free space (or medium) as a transmission path. In the delay interferometer disclosed in WO 2010-109640 A1, the length of an optical path is changed by disposing two prisms in two branched optical paths, respectively, and moving one of the prisms.
Further, JP 2009-300538 A discloses an optical phase shifting plate that uses a thermo-optical effect in a free space optical system. In the optical phase shifting plate disclosed in JP 2009-300538 A, an optical substrate capable of changing a refractive index with respect to transmitted light by the thermo-optical effect is mounted on a mounting portion formed in a bottom portion of a package, and a thin film heater is formed on a surface opposite a mounting portion in a surface of the optical substrate.
Since the polarization dependency of the thermo-optical effect is less than that of other physical optical phenomena, an optical phase shifter using the thermo-optical effect is preferable. Further, in the optical phase shifter using the thermo-optical effect, a phase can be shifted only by heat. Therefore, since a mechanism or the like mechanically moving an optical element is not necessary, miniaturization of the interferometer can be anticipated. However, in the free space optical system, when optical phase shift is performed using the thermo-optical effect and a temperature distribution occurs inside an element, a distribution of the refractive index occurs inside the element in response to the temperature distribution. Therefore, optical aberration may occur in response to the distribution of the refractive index. When the optical aberration occurs, coherency may deteriorate in the interferometer, and therefore the characteristics of the optical phase shifter may deteriorate.
When the technology disclosed in JP 2009-300538 A is applied, a thermo-optical element is disposed between the thin film heater serving as a heat source and the mounting portion of the package serving as a heat dissipation portion. Therefore, in the thermo-optical element, a temperature distribution may occur in a portion distant from the heat source or the heat dissipation portion. For this reason, the temperature distribution occurring in a region of an optical signal passing through the thermo-optical element may deteriorate the characteristics of the element. Further, for example, it is difficult to apply the technology disclosed in JP 2003-287632 A to the free space optical system. Therefore, when the area in which the thermo-optical element and the heat source have contact with each other is decreased, the response to changes in temperature is delayed. Thus, a new problem may arise in that practical phase shift may not be suitably realized.